With the rapid developments in aircraft technology, ever-increasing flight envelopes, and overall performance, the flight control systems implemented in modem aircraft have become extremely complex. Advanced flight control systems have been developed to address various aircraft characteristics such as flight performance, fuel efficiency, safety, etc. A fly-by-wire flight control system on modem aircraft typically includes a complex set of components including pilot sensors and controls, electronic processor, electronic wiring or data buses, actuators, and control surfaces.
In addition to primary control systems and control components, advanced aircraft often require a certain degree of redundancy in the control systems for safety requirements. The redundancy or backup system of a primary control system typically increases as the criticality of a control functions increases. Even with a carefully designed primary flight control system, it may be difficult to completely prevent so-called common mode failures within a control system, where an error or generic fault propagates from the primary control system to the redundant or backup components. A common mode failure also includes a generic fault that impacts all the identical redundant system elements simultaneously. As a consequence, redundant or backup control system may be configured as fully redundant and dissimilar, which unfortunately increases the part count or line-replaceable unit (LRU) count, cost, and weight for an aircraft. This problem can be especially difficult when the control system utilized a distributed fly-by-wire control system, where the actuators on the aircraft include their own servo loop closure electronics at or near the actuator.
A backup control system may vary between a completely redundant backup control system, duplicating the components and the performance of the primary control system, and a scaled down or minimum flight control system, reducing performance but saving on weight and cost. Because fully redundant backup control systems are expensive and often excessive, backup systems may be configured as simple as possible, making them robust and reliable while reducing cost and weight. Further, in order to prevent common mode failures, a backup system may be configured as independent and dissimilar, employing separate processors and flight computers for use in the event a failure in the primary control system.
On some advanced control systems for military aircraft, an active control stick in the cockpit may be used to actively shape the feel of the control stick by applying force or resistance on the control stick. The “active feel” of the active control stick or computer actuated control stick may be based on pilot inputs, aircraft configuration and flight conditions and may provide a pilot or copilot with improved situational awareness. For example, the improved situational awareness may include better coordination between pilots by electrically coupling the control sticks on each side of the cockpit, similar to the traditional cable driven coupling between control sticks. In addition to the pilot-to-pilot coupling, an active control stick can be made to follow the Autopilot commands so that the stick moves according to the Autopilot command inputs, giving the pilots better awareness of aircraft.
Other advanced features of an active control stick may include soft stops within the feel gradient of an active stick, which may be used to indicate various envelop and performance limits. For example, when the aircraft is approaching a stall condition, an active stick could incorporate a soft stop in the feel gradient to give a pilot a cue that he is approaching some predetermined margin (e.g. 15% from stall). Equivalently, a soft stop in the feel gradient could indicate an aircraft load factor limitation or attitude angle limitation. The pilot may then have the option to override such limits if he or she deems that appropriate. Yet another example of an advanced feature of an active control stick could be a variable amplitude and/or frequency stick shaker, which could be implemented so that the amplitude of the shaker function increases as the aircraft gets closer and closer to the stall angle of attack, for example.
To accomplish these active control stick functions, extensive processing may be required by stick control electronics to perform flight calculations and compute specific force loop functions, which dictate the amount and direction of force or resistance to apply to the control stick in a given flight condition. Although these calculations may be performed by the primary flight control electronics, such an arrangement may consume significant computing power needed for the primary control system. As such, typical active control sticks include independent and dedicated electronics and processors capable of performing the necessary computations for the active stick control, leaving the primary flight control electronics free to perform other flight critical functions.